The substrate processing apparatus is used to apply a surface treatment (e.g., etching, rinsing, etc.) to various substrates such as semiconductor wafers, glass substrates for photomasks, and glass substrates for liquid crystal. The substrate processing apparatus includes a tank for storing a treatment liquid (e.g., a chemical liquid such as an etching liquid or a rinse liquid), and supplies the treatment liquid in the tank to the surface to be treated of the substrate to process the substrate.
A heater is provided in the tank of the substrate processing apparatus for heating the treatment liquid in the tank to maintain the temperature of the treatment liquid above a desired temperature. In addition, an L-shaped pipe communicating with the inside of the tank is installed outside the tank to monitor the amount of liquid in the tank, and the liquid level in the pipe is detected by a sensor. For example, two liquid level sensors are provided to detect the upper limit liquid amount and the lower limit liquid amount in the tank. Various operations, such as to replenish the tank with a liquid and stop the replenishment, are controlled according to the detection results obtained by the liquid level sensors.
The temperature of the treatment liquid in the pipe is lower than the temperature of the treatment liquid in the tank having the heater therein. Therefore, when the temperature of the treatment liquid decreases, the saturation amount of the treatment liquid (the amount of an additive that can dissolve in the treatment liquid) decreases. As a result, the additive contained in the treatment liquid in the pipe precipitates, and adheres to the inner surface of the pipe. If the precipitated additive adheres to the inner surface of the pipe, the liquid level sensor may make an erroneous detection due to the precipitates. If the liquid level sensor make an erroneous detection, it becomes impossible to accurately figure out the desired amount of the treatment liquid in the tank, for example, the upper limit liquid amount and the lower limit liquid amount. As an example, when the lower limit liquid amount, i.e., the liquid shortage is not figured out due to the erroneous detection of the liquid level sensor, the treatment liquid is not supplied to the tank. This causes the heater to heat the empty tank, and may damage the apparatus.
The erroneous detection of the liquid level sensor is caused by the precipitates adhering to the inner surface of the pipe as described above. If the precipitates adhere to the inner surface of the pipe, for example, the liquid level sensor detects the treatment liquid even in the absence of the treatment liquid at the detection position for detecting the treatment liquid in the pipe, or keeps detecting the treatment liquid even when the treatment liquid in the pipe moves up and down around the detection position.